This invention relates to a master cylinder used in a vehicle hydraulic brake system, and particularly a master cylinder of the type including a cup received in a groove formed in the inner wall of a cylinder body and a piston for pressurizing brake fluid in a pressure chamber, the piston being in sliding contact with the radially inner periphery of the cup. This type of master cylinder is hereinafter sometimes referred to as a piston-to-cup slide type master cylinder.
Such piston-to-cup slide type master cylinders are disclosed in the following three patent publications: WO-02/064410 (Publication 1), JP patent publication 2000-71969 (Publication 2), and U.S. Pat. No. 4,249,381 (Publication 3).
The master cylinder disclosed in Publication 1 includes a primary cup received in a groove formed in the inner wall of the cylinder body for sealing the outer periphery of the piston. The primary cup includes a thick base portion having a lip extending forwardly from the base portion so as to be in sliding contact with the piston. The lip is adapted to be deformed under a pressure difference produced between the front and rear ends of the primary cup when the piston is returned quickly, thereby defining a fluid passage between the inner periphery of the primary cup and the piston. Through this fluid passage, brake fluid can flow forward.
The master cylinder disclosed in Publication 2 includes a spacer and a guide provided rearwardly of the primary cup. A fluid passage leading to the reservoir is formed between the spacer and the guide. The piston is formed with piston ports corresponding to this fluid passage.
The master cylinder disclosed in Publication 3 has a primary cup formed with grooves in its back surface. Brake fluid is supplied into the cylinder pressure chamber through these grooves, grooves formed in the piston at its tip, and a supply port.
One disadvantage of the master cylinder of Publication 1 is that the lip of the primary cup, which extends forwardly from its base portion, is low in strength.
In a hydraulic brake system including an electronic control unit which can perform automatic brake control such as vehicle stability control, the piston of the master cylinder may be pushed backward under the pressure of brake fluid flowing backward from the brake circuit provided between the wheels and the master cylinder. The lip of the primary cup has to have a sufficient strength so that it can withstand this pressure. Thus, the master cylinder disclosed in Publication 1 includes a reinforcing member such as a reinforcing ring for reinforcing the lip of the primary cup such that the lip can withstand the backflow pressure.
In some brake systems including an electronic control unit for controlling brake hydraulic pressure, brake fluid is sucked from the reservoir into the brake circuit through the master cylinder pressure chamber while the master cylinder is in its inoperative state. For this purpose, as disclosed in Publication 2, it is necessary to provide the piston with piston ports rearwardly of the primary cup so that the pressure chamber communicates with the reservoir through the piston ports while the master cylinder is in its inoperative state.
The sealing arrangement of the master cylinder disclosed in Publication 1, which practically comprises only the lip of the primary cup, is simpler in structure than that of the master cylinder disclosed in Publication 2, which comprises, besides the primary cup, the spacer and the guide. But in the arrangement of Publication 1, which needs the reinforcing member for reinforcing the lip, and in which the piston ports are provided rearwardly of the primary cup, the travel distance of the piston from its initial position to its advanced position where the piston ports are closed by the primary cup tends to be long. This deteriorates the driver's brake pedal feel.
The master cylinder disclosed in Publication 2 enjoys a shorter travel distance of the piston until the piston ports are closed. But in this arrangement, it is necessary that the cylinder body be assembled from a plurality of separate members in order to mount the spacer and the guide. This of course complicates the structure, increases the number of assembling steps and pushes up the cost. In Publication 3, the grooves formed in the back of the cup communicate with the pressure chamber not through the piston ports but through the axial grooves formed in piston at its tip. This arrangement also complicates the structure of the master cylinder.
An object of the invention is to provide a master cylinder which is simple in structure, which allows smooth suction of brake fluid from the reservoir into the pressure chamber through the primary cup when the piston is returned quickly, which allows smooth suction of brake fluid from the reservoir into the pressure chamber through the piston ports while the master cylinder is in its inoperative state, and which enjoys a shorter travel distance of the piston until the piston ports are closed.